The nonzero roots of the equation $x^2 + 6x + k = 0$ are in the ratio $2:1$. What is the value of $k$?
By Vieta's formulas, the sum of the roots is $-6.$  Since they are in the ratio $2:1,$ the roots are $-4$ and $-2.$  Then $k$ is their product, namely $(-4)(-2) = \boxed{8}.$